Second seeded Frenchman Gilles Simon stayed on course for winning his maiden ATP world tour title this season with a hard-fought 4-6, 6-3 6-4 win over Russian Evgeny Korolev to reach the semifinals of the 2009 PTT Thailand Open.

Simon fired 7 aces and converted 3 out of 5 break point opportunities to win the thriller in a match that lasted one hour and 43 minutes.

Simon's opponent in the semifinal is the Austrian Jurgen Melzer, the No.6 seed in the tournament, who, earlier in the day, took out unseeded German Andreas Beck 6-4, 6-2 in just 66 minutes to advance to the second semifinal of the year.

In another quarterfinal match from the evening session, top seeded Frenchman Jo-Wilfried Tsonga registered a come-from-behind victory over Swiss qualifier Marco Chiudinelli to reach the last four.

Tsonga lost the opening set in a closely fought tie-break but came back to win the next two with the help of two crucial breaks, one in each set, to close the two hour and 21 minute encounter. 

Tsonga's opponent in the semifinal is yet to be decided as the day's last match between Serbian Viktor Troicki and American John Isner is scheduled for a late evening start.